Digital terminals are connected to telephone switching systems by either special digital port circuits or modems which interface the digital terminal to any standard analog port circuit. In either case, a standard telephone station set is required to originate a data call for the digital terminal. This telephone station set is bridged on to the communication pair serving the digital terminal and is used to originate a data call by implementing the off-hook and digit dialing functions. Once the data call has been originated in this fashion, the telephone station set is returned to the on-hook condition and remains unused for the remainder of the data call.
Thus, each user of a digital terminal is generally provided with two communication pairs: one equipped with a telephone station set--used for voice calls; one equipped with a telephone station set and a digital terminal--used for data calls. It is apparent that such an arrangement is an inefficient use of equipment.